The cyclotron resonance maser (or gyrotron) class of device has been demonstrated to be an efficient means for generating r.f. power at millimeter wavelengths without intricate r.f. circuitry. In brief, the principle of operation is that electrons of a hollow monoenergetic beam whose cyclotron frequency is determined by a strong, uniform axial magnetic field interact mainly with the transverse r.f. fields of a traveling wave within a cylindrical waveguide. Power extraction from the electron beam occurs near the waveguide mode dispersion curve intersection with the Doppler shifted cyclotron frequency.
A crucial aspect of the performance characteristics of a gyroamplifier is the quality of the beam formation/focusing system. The system must successfully deliver a substantially monoenergetic beam (low axial velocity spread for high efficiency) with a high rotational energy component through a long radio frequency interaction zone (25 to 50 cyclotron periods) in a strong magnetic field (0.4 to 1.8T depending upon harmonic). The beam system then must properly distribute both the modulated and unmodulated beam at an externally-cooled collector surface at tolerable power density levels (&lt;3 KW/cm.sup.2). The radial spatial distribution of the beam should favor location near the field antinodes of the mode of interest in order to selectively minimize interaction with undesired modes. As a practical matter, it is difficult to satisfy all these requirements for a given set of beam parameters.
The most successful method in the prior art for accomplishing a workable compromise among the desired beam parameters has been the so-called "magnetron injection gun" (MIG). While the MIG type gun provides adequate performance for gyrotron oscillators, the gun appears to provide only marginal performance for the much longer interaction lengths required for gyroamplification at high gain. The deficiency of the MIG-type gun has been identified as excessive axial velocity spread which adversely impacts upon electronic efficiency.
The geometries associated with the MIG-type gun provide proper boundary conditions for launching a narrow strip beam. However, there are several complicating restrictions. The current density of practical electron emitters has an upper bound at present of approximately 8 A/cm.sup.2. The surface roughness, the initial thermal velocity of the emitted electrons and the space charge of the beam, especially at low velocity, creates significant trajectory distortions which translate into velocity spread in the launched beam. This has necessitated adoption of temperature-limited operation in order to decrease the transit times during beam launch by providing locally high fields at the emitter surface in the order of 10.sup.5 -10.sup.6 V/cm.
There have been other attempts to improve the beam formation situation by employing alternative schemes such as field reversals and bifilar focusing of Pierce-type guns operated at high mirror ratios. A formulation of the shielded center-pole gun, employing field reversal, has been described in "Magnetically-Shielded Electron Guns with a Center Magnetic Post," N. R. Vanderplaats, H. E. Brown and S. Ahn, Technical Digest IEDM, pp. 336-338, Wash. (1981).
Electron beams suitable for gyrotron type of r.f. interaction require electron guns that differ substantially from those employed in conventional O-type microwave tubes. Because power conversion involves the rotational kinetic power of the gyrobeam, beam formation for this newer class of devices must generate a transverse-to-axial velocity ratio, .alpha., typically between 1.0 and 2.0 for efficient operation. Additionally, in order to provide better performance, it is desirable for the longitudinal velocity spread to be kept small (less than 20% for oscillators and less than 5% for amplifiers).
It is therefore an object of this invention to provide a new type of gyrobeam electron gun which meets the abovestated requirements.
It is a further object of this invention to provide a novel beam formation scheme which circumvents most of the limitations associated with the prior magnetically-immersed gun configurations.
It is a feature of this invention that there is a separation of the electrostatic beam formation problem from the magnetic focusing problem.